Broadly speaking, the original definition of the metre was

"one ten-millionth of the distance
from the North Pole to the Equator
on the meridian running through Paris."

This was adopted in competition with the definition of

"the pendulum length giving
a half period of one second"

The definitions are remarkably similar because of the coincidence that $\pi^2$ is very nearly equal to g, acceleration due to gravity.

However, both definitions have problems. You can read more about them in the Wikipedia article quoted below.

More recently, the metre has been redefined as:

"the distance travelled by light in vacuum during
a time interval of 1/299 792 458 of a second."

Thus the speed of light is now, by definition, 299 792 458 m/s.

Further reading:
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